Apparatus and method of measuring a property of a substrate

ABSTRACT

The present invention makes the use of measurement of a diffraction spectrum in or near an image plane in order to determine a property of an exposed substrate. In particular, the positive and negative first diffraction orders are separated or diverged, detected and their intensity measured to determine overlay (or other properties) of exposed layers on the substrate.

FIELD

The present invention relates to methods of inspection usable, forexample, in the manufacture of devices by lithographic techniques and tomethods of manufacturing devices using lithographic techniques.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor the lithographic process, it is desirable to measureparameters of the patterned substrate, for example the overlay errorbetween successive layers formed in or on it. There are varioustechniques for making measurements of the microscopic structures formedin lithographic processes, including the use of scanning electronmicroscopes and various specialized tools. One form of specializedinspection tool is a scatterometer in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered or reflected beam are measured. By comparing theproperties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometer are known.Spectroscopic scatterometers direct a broadband radiation beam onto thesubstrate and measure the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. Angularly resolved scatterometers use a monochromatic radiationbeam and measure the intensity of the scattered radiation as a functionof angle.

As mentioned above, a pattern is created on the surface of a substrate,this pattern representing the IC, or whatever product is being made onthe substrate. The way that the pattern is made is that repeated layersof resist are laid on the substrate, then exposed, then washed or bakedor other such post-exposure processes. It is desirable that each of thelayers that is to be exposed be aligned with the layer below it suchthat the pattern builds up as accurately as possible, ensuring effectiveelectrical connections where required and also enabling smaller andsmaller products to be created while avoiding cross-talk betweenneighboring structures. Scatterometers are useful in determining whetheror not subsequent layers are aligned as they should be. The alignment ofsubsequent layers is known as overlay. An error in overlay means that alayer is offset with respect to a layer below (or indeed above) it. If alayer is offset with respect to a layer below it, the quality ofelectrical contact between structures within these respective layerswill be reduced. In extreme cases, complete loss of electrical contactor short-circuiting might occur. The same is true if a product layer isrotated with respect to the product layer below it. These types ofoverlay error may be measured using scatterometers.

In order to measure and thereby correct for (or, preferably, prevent)overlay errors, test patterns are created on substrates which have knownproperties and which are tested using a radiation beam and ascatterometer detector, which measures the diffraction spectra of theradiation beam that has reflected from the test pattern. In order toreduce the amount of space taken up by these test patterns, the testpatterns are generally created in the scribe lanes between dies andfields on the substrate. It is these scribe lanes that will subsequentlybe sawn in order to separate the various products, and so are not usefulfor product. The test patterns are generally known as overlaymeasurement targets or overlay targets. However, the scribe lanes aresmall (to leave more room for product) and are also generally packedwith a large range of test patterns for various purposes. Because of therelatively large size of the overlay targets, for on-product overlaymeasurement (i.e. measurement on substrates that also contain product,as opposed to test substrates that may be used purely for testing), inpractice, the targets are in the scribe lanes between the dies. However,this means that models are used to predict real product overlay becausethe real product overlay is not being measured directly.

Models that are used in interpolating scribe lane overlay targets toin-die product are approximations. These approximations may have someerrors. The errors may be made worse if fewer targets are available inthe scribe lane. It is often the case that fewer than an optimum numberof overlay targets are available because the scribe lane “real estate”is valuable to users of lithographic apparatuses (as mentioned above).

To minimize above-mentioned interpolation errors, it is desirable to beable to measure overlay on in-die targets. Because substrate “realestate” is at a premium inside the dies, it is desirable that theseoverlay targets be as small as possible, and preferably 10 by 10 micronsor less. However, with very small targets, there are stringent sizerequirements which may lead to signal-to-noise ratio problems.Furthermore, signal-to-noise ratios and cross-talk between the overlaytarget and neighboring product structure may be undesirable. This isbecause the illuminating beam will be limited to the size that it can befocused down to, which may be larger than the target itself because ofthe desired extremely small target size.

SUMMARY

It is desirable to provide a technique for measuring overlay that can becarried out on small in-die overlay targets using available alignmentradiation beams.

According to an aspect of the invention, there is provided an inspectionapparatus, scatterometer, lithographic apparatus or lithographic cellconfigured to measure a property of a substrate, including: a radiationsource configured to output a radiation beam, the radiation beamdirected to a surface of the substrate; a collimator configured tocollimate and focus the radiation beam once reflected from the surfaceof the substrate, and to separate diffraction orders of the reflectedradiation beam; and a detector configured to detect an angle-resolvedspectrum of the reflected radiation beam, wherein the property of thesubstrate is measured by measuring a property of the reflected spectrumin a plane positioned in a range from where the radiation beam isconverging toward an image plane of the collimator to where theradiation beam is diverging from the image plane.

According to another aspect of the present invention, there is provideda method of measuring a property of a substrate, including irradiating atarget on a substrate with a radiation beam; reflecting the radiationbeam from the target; collimating and focusing the reflected radiationbeam using an optical system; diverging first diffraction orders of thereflected radiation beam; detecting the intensity of the firstdiffraction orders in a plane positioned within a range from where thereflected radiation beam is converging toward an image plane of theoptical system to where the reflected radiation beam is diverging fromthe image plane; and determining, from the intensity of the firstdiffraction orders, a property of the target of the substrate.

According to yet another aspect of the invention, there is provided adevice manufacturing method including using a lithographic apparatus toform a pattern on a substrate; and determining a value related to aparameter of the pattern printed by: irradiating a target on a substratewith a radiation beam; reflecting the radiation beam from the target;collimating and focusing the reflected radiation beam using an opticalsystem; diverging first diffraction orders of the reflected radiationbeam; detecting the intensity of the first diffraction orders in a planepositioned within a range from where the reflected radiation beam isconverging toward an image plane of the optical system to where thereflected radiation beam is diverging from the image plane; anddetermining, from the intensity of the first diffraction orders, aproperty of the target of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus in accordance with an embodimentof the invention;

FIG. 2 depicts a lithographic cell or cluster in accordance with anembodiment of the invention;

FIG. 3 depicts a scatterometer in accordance with an embodiment of theinvention;

FIG. 4 depicts a scatterometer in accordance with an embodiment of theinvention;

FIG. 5 depicts an illumination set-up according to an embodiment of theinvention;

FIG. 6 depicts an illumination set-up according to an embodiment of theinvention; and

FIG. 7 depicts an illumination set-up according to an embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g. UV radiation or DUV radiation); a patterningdevice support or support structure (e.g. a mask table) MT constructedto support a patterning device (e.g. a mask) MA and connected to a firstpositioner PM configured to accurately position the patterning device inaccordance with certain parameters; a substrate table (e.g. a wafertable) WT constructed to hold a substrate (e.g. a resist-coated wafer) Wand connected to a second positioner PW configured to accuratelyposition the substrate in accordance with certain parameters; and aprojection system (e.g. a refractive projection lens system) PLconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. including one or moredies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask table)MT, and is patterned by the patterning device. Having traversed thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PL, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g. an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g. mask) MA withrespect to the path of the radiation beam B, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g. mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the patterning device support (e.g. mask table) MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device (e.g.mask) MA and substrate W may be aligned using mask alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device (e.g. mask) MA, themask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g. mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed. In step mode, the maximum size of theexposure field limits the size of the target portion C imaged in asingle static exposure.

2. In scan mode, the patterning device support (e.g. mask table) MT andthe substrate table WT are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e. a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the patterning device support (e.g. masktable) MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g. mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WT is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, adjustments may be made to exposures ofsubsequent substrates, especially if the inspection can be done soon andfast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped andreworked—to improve yield—or discarded, thereby avoiding performingexposures on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the inspectionapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB), whichis customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

FIG. 3 depicts a scatterometer SM1 which may be used in an embodiment ofthe present invention. It includes a broadband (white light) radiationprojector 2 which projects radiation onto a substrate W. The reflectedradiation is passed to a spectrometer detector 4, which measures aspectrum 10 (intensity (I) as a function of wavelength (λ)) of thespecular reflected radiation. From this data, the structure or profilegiving rise to the detected spectrum may be reconstructed by processingunit PU, e.g. by Rigorous Coupled Wave Analysis and non-linearregression or by comparison with a library of simulated spectra as shownat the bottom of FIG. 3. In general, for the reconstruction the generalform of the structure is known and some parameters are assumed fromknowledge of the process by which the structure was made, leaving only afew parameters of the structure to be determined from the scatterometrydata. Such a scatterometer may be configured as a normal-incidencescatterometer or an oblique-incidence scatterometer.

Another scatterometer SM2 that may be used with an embodiment of thepresent invention is shown in FIG. 4. In this device, the radiationemitted by radiation source 2 is focused using lens system 12 throughinterference filter 13 and polarizer 17, reflected by partiallyreflected surface 16 and is focused onto substrate W via a microscopeobjective lens 15, which has a high numerical aperture (NA), preferablyat least 0.9 and more preferably at least 0.95. Immersion scatterometersmay even have lenses with numerical apertures over 1. The reflectedradiation then transmits through partially reflective surface 16 into adetector 18 in order to have the scatter spectrum detected. The detectormay be located in the back-projected pupil plane 11, which is at thefocal length of the lens system 15, however the pupil plane may insteadbe re-imaged with auxiliary optics (not shown) onto the detector. Thepupil plane is the plane in which the radial position of radiationdefines the angle of incidence and the angular position defines azimuthangle of the radiation. The detector is preferably a two-dimensionaldetector so that a two-dimensional angular scatter spectrum of asubstrate target 30 can be measured. The detector 18 may be, forexample, an array of CCD or CMOS sensors, and may use an integrationtime of, for example, 40 milliseconds per frame.

A reference beam is often used for example to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the beam splitter 16 part of it is transmitted through the beamsplitter as a reference beam towards a reference mirror 14. Thereference beam is then projected onto a different part of the samedetector 18.

A set of interference filters 13 is available to select a wavelength ofinterest in the range of, say, 405-790 nm or even lower, such as 200-300nm. The interference filter may be tunable rather than including a setof different filters. A grating could be used instead of interferencefilters.

The detector 18 may measure the intensity of scattered radiation at asingle wavelength (or narrow wavelength range), the intensity separatelyat multiple wavelengths or integrated over a wavelength range.Furthermore, the detector may separately measure the intensity oftransverse magnetic- and transverse electric-polarized radiation and/orthe phase difference between the transverse magnetic- and transverseelectric-polarized radiation.

Using a broadband radiation or light source (i.e. one with a wide rangeof light frequencies or wavelengths—and therefore of colors) ispossible, which gives a large etendue, allowing the mixing of multiplewavelengths. The plurality of wavelengths in the broadband preferablyeach has a bandwidth of δλ and a spacing of at least 2δλ (i.e. twice thebandwidth). Several “sources” of radiation can be different portions ofan extended radiation source which have been split using fiber bundles.In this way, angle resolved scatter spectra can be measured at multiplewavelengths in parallel. A 3-D spectrum (wavelength and two differentangles) can be measured, which contains more information than a 2-Dspectrum. This allows more information to be measured which increasesmetrology process robustness. This is described in more detail inEP1,628,164A.

The target 30 on substrate W may be a grating, which is printed suchthat after development, the bars are formed of solid resist lines. Thebars may alternatively be etched into the substrate. This pattern issensitive to chromatic aberrations in the lithographic projectionapparatus, particularly the projection system PL, and illuminationsymmetry and the presence of such aberrations will manifest themselvesin a variation in the printed grating. Accordingly, the scatterometrydata of the printed gratings is used to reconstruct the gratings. Theparameters of the grating, such as line widths and shapes, may be inputto the reconstruction process, performed by processing unit PU, fromknowledge of the printing step and/or other scatterometry processes.

As mentioned above, the scatterometers of FIGS. 3 and 4 may be used tomeasure the overlay of subsequent exposure layers. The overlay targetsthat are used for this process are often in the form of periodicgratings. By illuminating this grating first from one direction (at aspecific angle of incidence and azimuthal angle) and subsequently fromthe opposite direction (e.g. same angle of incidence but oppositeazimuthal angle), diffraction spectra are created and may be detected,for example in the pupil plane of the objective lens 12.

Illuminating the target first from one direction then from another givesrise to two measurable intensities: one for each radiation beam that hasbeen reflected from each direction. These intensities may then besubtracted from a target image formed by first-order diffracted rays(also produced from the target on the surface of the substrate). Thesubtraction of the individual intensities from a target image will giverise to an intensity image that shows any asymmetry in the target as avariation in intensity.

However, this method requires two serial exposures with an aperturechange in between the exposures. This reduces throughput and mayintroduce additional error sources (e.g. if the illuminating radiationused in the first direction is not precisely the same as that used inthe second direction).

It is therefore proposed here to measure small in-die overlay targets(e.g. 10 by 10 microns) with an image-based technique using a relativelylarge measurement spot. A measurement spot is the size of the focusedilluminating beam. The measurement spot of an embodiment of the presentinvention may be significantly larger than the overlay target itself.When using an image-based technique, a detector is positioned in theimage plane of the optical system that is used to collimate and focusthe light reflected from the overlay target. The optical system may bebroadly termed hereinafter a collimator. The image plane is a plane inwhich an image is refocused of an object (i.e. the overlay target inthis case) from which radiation has been reflected. If a detector isplaced in this image plane, a sharp image of the object will bedetected. This is in contrast to other methods, where a detector isplaced in a pupil plane of the optical system that is used to collimateand focus radiation reflected from the object to be inspected.

Because of its different position in the collimator, the detector of anembodiment of the present invention may be different from otherdetectors used in existing scatterometers. Alternatively, existingdetectors may be moved, or the optics adjusted so that the image planeof the optical system is positioned at the detector surface.

FIG. 5 shows an embodiment of the present invention. A radiation beam isreflected from an overlay target 30 which is positioned on the substrateW. The target may be a grating or other shape that is easilyreconstructed or useful for investigating asymmetries in radiationdiffracted from it. The overlay target may be in-die; i.e. within theproduct area rather than in the scribe lanes, thus giving a moreaccurate indication of the overlay of the product area itself. Thetarget may be very small: smaller than the measurement spot of theradiation beam.

In this embodiment, the optical system or collimator includes a firstobjective lens 40 that can collimate the diverging beam which has beenreflected from the object 30 of the substrate W. A subsequent lens onthe other side of a pupil plane PP focuses the reflected radiation beamand causes it to converge to a point in an image plane on the opticalaxis. The radiation beam, unimpeded, diverges and is again collimated bylens 50.

This reflecting, collimating and focusing of the radiation beam that iscarried out by the optical system or collimator gives rise to adiffraction spectrum that is divided into diffraction orders, with thezeroth order in the centre of the radiation beam and progressivelyhigher orders positioned radially outwards. The diffraction spectrum isknown as an “angle-resolved” spectrum. As long as the objective lens 40has a high enough numerical aperture to capture and collimate up to atleast the first diffraction orders of the diffracted radiation, theincident radiation may be incident at a large range of angles. Thenumerical aperture of the objective lens 40 may be increased to over 0.9and even over 1 with the introduction of a liquid (such as water)between the substrate W and the objective lens 40. This is referred toas “immersion” of the system, as described above.

A spatial filter 60 may be arranged in the second pupil plane of theoptical system or collimator. The purpose of the spatial filter 60 is tostop the zeroth diffraction order of the reflected radiation, which isin the center of the diffraction spectrum, from reaching the detectorCCD. This creates “darkfield” conditions, thus increasing contrast inthe detected image and making asymmetries in the first orders morevisible. The spatial filter aids in removing all radiation other thanthat which has been scattered from an object (i.e. the target 30) whichis to be measured. This enables the measurement spot to be larger thanthe target 30 because non-scattered radiation from an area surroundingthe target can be filtered out using the spatial filter 60.

The first diffraction order, which is radially outside of the zerothorder, is allowed to pass through the spatial filter 60. Preferably,only the first diffraction orders are transmitted through the spatialfilter 60. Higher diffraction orders should also be also stopped by thespatial filter 60. Each of the positive and negative first diffractionorders that pass through the spatial filter may be subsequently divergedby respective optical wedges 70. This allows the separation of the twofirst order diffracted beams from each other so that they create a“double vision” effect 100 and are separately detected at the detectorCCD. These first order diffracted rays may be subsequently focused by afurther lens 80 in order to provide a further image plane IP where theimages of the target 30 in the separated first diffraction orders willbe focused and sharp. At this image plane IP, the first diffractionorders impinge on the detector CCD.

To prevent these two first diffraction orders from merging into a singleimage on the camera, an optical wedge 70 may be introduced into thepupil plane (after the spatial filter 60) and into the path of thepositive and negative first diffraction orders. Alternatively, wedgeswith opposite (or at least different) tilts may be applied to bothdiffraction orders. The optical wedge gives rise to a shift between thetwo images from the +1 and −1 first diffraction orders such that the twoimages are separately detected on an image plane IP, but not(necessarily) on the optical axis. This is known as “double-vision”measurement 100.

The double vision effect 100 is caused because the radiation does notconverge on the central optical axis of the optical system or collimatorto form a single image. Rather, each of the positive and negative firstdiffraction orders are individually focused using the lens 80 ontoseparate positions on the detector CCD, thus creating two focused images100. The two separate images may then be compared.

Both positive and negative first diffraction orders are preferablydetected simultaneously, as shown in FIG. 5. In this way, two images arecreated at the detector of the target 30 from the substrate W. Thedetected information 100 is shown at the bottom of FIG. 5. On the leftis the +1 diffraction order and on the right is the −1 diffractionorder. It will be appreciated that, depending on the optical system orcollimator, these may be reversed or displayed in a different way.

The intensities of the two images 100 can be compared and the asymmetryin the measured intensities can be linked to the overlay error in thetarget 30.

Compared to illuminating the overlay target first from one direction andthen from another direction, image-based measurement such as thatdescribed above has three major benefits. The first is higherthroughput. Because different measurements of the same target may bemeasured simultaneously, measurement of overlay may be carried out morequickly and results fed back to the lithography exposure system, thusincreasing the speed at which correctly-exposed substrates may becreated. The second benefit is that the system of an embodiment of thepresent invention is less sensitive to variations between serialexposures. This is because only one exposure of the overlay target 30 isrequired. A third benefit is that the +1 and −1 diffraction orders aregenerated by one common incident ray (rather than by two only partlyrelated incident rays). The illuminating radiation is thereforeconstant, thus giving rise to fewer sources of measurement error.

As a refinement of the method described above, vision analysis can beapplied to find accurately the target images in the detector picture. Inother words, analysis of the detector signal may be used to determineexactly where on the substrate W the overlay target 30 exists withrespect to the optical axis of the sensor optics, and this may be doneautomatically. This reduces the need for very precise positioning of thedetector and the alignment beam with respect to the substrate and itstarget. Errors of relative positioning may thereby be automaticallyremoved from the overlay calculation. Alternatively, this technique maybe used to generate a warning signal to indicate that the positioning ofthe target with respect to the sensor is not sufficiently good, and thatrepositioning is needed (including information on direction andmagnitude of the repositioning step). This repositioning may also beautomated.

FIG. 6 shows an embodiment of the present invention. The common featuresare labeled with common reference numerals. The basic set-up is thesame, with an objective lens 40 collimating the reflected radiation andthe radiation subsequently being focused and re-collimated by lens 50.However, there is no spatial filter (60) in this embodiment for blockingthe zeroth order radiation. However, a system of optical wedges 72 isincluded, which allows three separate target images 102 to be formed asshown in the bottom of FIG. 6.

In the specific embodiment shown, the zeroth and first orders aredetected by the detector CCD. However, it is within the scope of theinvention to detect higher-order diffracted rays as well. The opticalwedges 72 serve to separate out the zeroth diffraction order, which canthen be removed from calculations of overlay. The overlay can becalculated again from the two target images from the respective +1 and−1 first-order (and/or higher-order) diffracted rays in the same way asin the first embodiment.

An embodiment of the present invention is shown in FIG. 7. As was thecase for the embodiment of FIG. 6, common features with the firstembodiment are depicted with common reference numerals. In theembodiment of FIG. 7, the zeroth order diffracted radiation is againblocked using the spatial filter 60, and collimation and focusing of thereflected radiation is carried out in the same way as in the embodimentsof FIGS. 5 and 6. However, no optical wedges (70, 72) are used. Instead,the measuring detector CCD is placed not in an image plane IP, but in anintermediate plane between the pupil plane PP,60 and the image plane IP,wherein the radiation is converging from its parallel beam at the pupilplane to a focused point at the image plane IP. Alternatively, andequally validly, the detector CCD may be placed downstream of the imageplane IP, wherein the radiation is diverging again after having focusedon the image plane IP. Either method results in two spatially separated,blurred images 104 of the target 30.

The embodiment shown in FIG. 7 does not use optical wedges to create thedouble vision effect (100, 102), but instead, a second set of lenses 82is used to focus the first diffraction orders toward an image plane IPthat is on the optical axis. The blurred target images are separated bythe fact that they have not converged together on the axis yet (or have,but are diverging again), rather than by the inclusion of optical wedgesto force a focus on an axis other than the main optical axis of theoptical system or collimator. The intensity of the blurred images can bemeasured even though the images are not in focus. The benefit of thisembodiment is therefore that the added hardware of optical wedges is notrequired, and the optical system or collimator can be simplified andmanufactured more cheaply.

However, optical wedges may be used as a variation in this embodiment,and the detector may be placed wherever the separated first orders aremeasurable, whether they are in focus or not.

Again in the embodiment of FIG. 7 or its variation, overlay iscalculated from the difference in average intensity of these blurredtargets 104.

As a refinement of the embodiment of FIG. 7, a second spatial filter 62may be employed in an intermediate image plane (which is conjugate tothe object plane; i.e. on the same optical axis as the plane in whichthe object 30 is situated and therefore producing an image with alldiffraction orders, which is a full-data image, of the object 30) toprevent rays originating from outside the actual overlay target to beincluded in the blurred target image. This helps to prevent noise fromaffecting the intensity measurement at the detector CCD. Furthermore,this enables the measurement spot to be larger than the target on thesubstrate, enabling the target to be smaller. The first spatial filter60 may also contribute to this benefit.

The skilled person will be able to envisage within the scope of theclaims other set-ups of the apparatus which will enable a detector, ordetector array, to be in or near an image plane and to use first (orhigher) diffraction orders that are separated from each other to obtainseparate signals or images containing only data from the first (and/orhigher) diffraction orders. From a comparison of these separate signalsor images, an overlay measurement may be determined.

The skilled person will also appreciate that other properties of thesubstrate may be measured using the techniques described. For example,measurement parameters of an object on the substrate may bereconstructed, such as critical dimension (CD), object height, sidewallangle of the object with respect to the substrate, etc., or alignment ofthe substrate with respect to an external constant may be measured.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

The invention claimed is:
 1. An inspection apparatus configured todetermine a property of a substrate, comprising: a radiation sourceconfigured to output a radiation beam toward an overlay target on asurface of the substrate; a collimator configured to collimate and focusthe radiation beam once reflected from the overlay target, and toseparate diffraction orders of a diffraction spectrum of the reflectedradiation beam; a detector configured to detect at least a subset of theseparated diffraction orders of the reflected radiation beam and todetermine first and second properties of respective first and secondportions of the subset; and a processor configured to: determine theproperty of the substrate by comparing the first and second propertiesin a plane positioned in a range from where the radiation beam isconverging toward an image plane of the collimator to where theradiation beam is diverging from the image plane; determine a positionof the overlay target with respect to an optical axis of the collimator;and remove an error of relative positioning from the determination ofthe property of the substrate based on the determined position of theoverlay target.
 2. The inspection apparatus according to claim 1,wherein the plane is positioned between an image plane and a pupil planeof the collimator, where, in use, the reflected radiation beam convergestowards the image plane.
 3. The inspection apparatus according to claim1, wherein the plane is positioned downstream of a plane conjugate tothe image plane, where, in use, the reflected radiation beam divergesfrom the image plane.
 4. The inspection apparatus according to claim 1,wherein the plane is positioned on an image plane of the collimator. 5.The inspection apparatus according to claim 1, wherein the plane ispositioned near an image plane of the collimator.
 6. The inspectionapparatus according to claim 1, further comprising a spatial filterbetween the substrate and the detector that is configured to block azeroth diffraction order of the diffraction spectrum.
 7. The inspectionapparatus according to claim 1, further comprising an optical deviceconfigured to spatially diverge positive and negative first diffractionorders of the diffraction spectrum before the spectrum is detected bythe detector.
 8. The inspection apparatus according to claim 1, whereinthe detector is arranged to detect positive and negative firstdiffraction orders of the diffraction spectrum substantiallysimultaneously.
 9. The inspection apparatus according to claim 1,wherein the property of the substrate to be measured is the overlay of afirst layer with respect to a second layer on the substrate.
 10. Theinspection apparatus according to claim 9, wherein: the first measuredproperty comprises a positive first diffraction order and the secondmeasured property comprises a negative first diffraction order; and theoverlay is measured by comparing intensities of the positive andnegative first diffraction orders.
 11. The inspection apparatusaccording to claim 1, wherein the property of the substrate comprises ameasurement parameter of the overlay target.
 12. The inspectionapparatus according to claim 1, wherein the collimator comprises anoptical device configured to separate the diffraction spectrum into twoor more sub-beams, at least two of the sub-beams comprising non-zerodiffraction orders of the reflected beam.
 13. The inspection apparatusaccording to claim 12, wherein: the at least two sub-beams comprise apositive and a negative first diffraction order respectively; the subsetof the separated diffraction orders detected by the detector comprisesthe positive and negative first diffraction orders of the at least twosub-beams; and the processor is configured to determine, from anintensity of each of the two first diffraction orders detected by thedetector, an overlay of a first layer on the substrate with respect to asecond layer.
 14. The inspection apparatus according to claim 1, furthercomprising a spatial filter positioned in an intermediate image plane ofthe collimator, the spatial filter being configured to prevent strayradiation from reaching the detector, the stray radiation originatingfrom outside of a target on the substrate from which the radiation beamis reflected.
 15. The inspection apparatus according to claim 1, whereinthe collimator includes an area between the substrate and a lens that isfilled with a fluid configured to create a higher effective numericalaperture of the lens.
 16. The inspection apparatus according to claim 1,wherein the detector is configured to detect the diffracted spectrum fora plurality of wavelengths and a plurality of incident angles of theradiation beam substantially simultaneously.
 17. The inspectionapparatus according to claim 1, wherein the detector is configured tomeasure defocused images of the surface of the substrate and theproperty to be measured is determined by analyzing the defocused images.18. The inspection apparatus according to claim 1, wherein; the detectoris further configured to output the detected at least subset of theseparated diffraction orders as a detector signal.
 19. A scatterometerconfigured to determine a property of a substrate, comprising: aradiation source configured to output a radiation beam toward an overlaytarget on a surface of the substrate; a collimator configured tocollimate and focus the radiation beam once reflected from the overlaytarget, and to separate diffraction orders of an angle-resolved spectrumof the reflected radiation beam; and a detector configured to detect theangle-resolved spectrum of the reflected radiation beam and to determinefirst and second properties of the angle-resolved spectrum, the detectorbeing situated in a plane positioned in a range from where the radiationbeam is converging toward an image plane of the collimator to where theradiation beam is diverging from the image plane; and a processorconfigured to: determine the property of the substrate by comparing thefirst and second properties of the angle-resolved spectrum; determine aposition of the overlay target with respect to an optical axis of thecollimator; and remove an error of relative positioning from thedetermination of the property of the substrate based on the determinedposition of the overlay target.
 20. A method of measuring a property ofa substrate, comprising: irradiating a target on a substrate with aradiation beam, the radiation beam being reflected from the target as areflected radiation beam; collimating and focusing the reflectedradiation beam using a collimator; diverging first diffraction orders ofthe reflected radiation beam; detecting the intensity of the firstdiffraction orders in a plane positioned within a range from where thereflected radiation beam is converging toward an image plane of thecollimator to where the reflected radiation beam is diverging from theimage plane; comparing the intensity of each of the first diffractionorders against the other; determining, from the comparing of theintensity of the first diffraction orders, a property of the substrate;determining a position of the target with respect to an optical axis ofthe collimator; and removing an error of relative positioning from thedetermining of the property of the substrate based on the determining ofthe position of the target.
 21. The method according to claim 20,further comprising blocking a zeroth diffraction order from thereflected radiation beam.
 22. A lithographic apparatus comprising: anillumination system arranged to illuminate a pattern; a projectionsystem arranged to project an image of the pattern onto a substrate; andan inspection apparatus configured to determine a property of thesubstrate, the inspection apparatus comprising: a radiation sourceconfigured to output a radiation beam toward an overlay target on asurface of the substrate; a collimator configured to collimate and focusthe radiation beam once reflected from the overlay target, and toseparate diffraction orders of a diffraction spectrum of the reflectedradiation beam; a detector configured to detect at least a subset of theseparated diffraction orders of the reflected radiation beam and todetermine first and second properties of respective first and secondportions of the subset; and a processor configured to: determine theproperty of the substrate by comparing the first and second propertiesin a plane positioned in a range from where the radiation beam isconverging toward an image plane of the collimator to where theradiation beam is diverging from the image plane; determine a positionof the overlay target with respect to an optical axis of the collimator;and remove an error of relative positioning from the determination ofthe property based on the determined position of the overlay target. 23.A lithographic cell comprising: a coater arranged to coat a substratewith a radiation sensitive layer; a lithographic apparatus arranged toexpose images onto the radiation sensitive layer of the substrate coatedby the coater; a developer arranged to develop images exposed by thelithographic apparatus; and an inspection apparatus configured todetermine a property of the substrate, the inspection apparatuscomprising: a radiation source configured to output a radiation beamtoward an overlay target on a surface of the substrate; a collimatorconfigured to collimate and focus the radiation beam once reflected fromthe overlay target, and to separate diffraction orders of a diffractionspectrum of the reflected radiation beam; a detector configured todetect at least a subset of the separated diffraction orders of thereflected radiation beam in a plane positioned in a range from where theradiation beam is converging toward an image plane of the collimator towhere the radiation beam is diverging from the image plane; and aprocessor configured to: determine first and second properties ofrespective first and second portions of the subset, and the property ofthe substrate by comparing the first and second properties; determine aposition of the overlay target with respect to an optical axis of thecollimator; and remove an error of relative positioning from thedetermination of the property of the substrate based on the determinedposition of the overlay target.
 24. A device manufacturing methodcomprising: forming a pattern on a substrate using a lithographicapparatus; and determining a value related to a parameter of the patternformed by: irradiating a target on the substrate with a radiation beam,the radiation beam being reflected from the target as a reflectedradiation beam; collimating and focusing the reflected radiation beamusing a collimator; diverging first diffraction orders of the reflectedradiation beam; detecting the intensity of the first diffraction ordersin a plane positioned within a range from where the reflected radiationbeam is converging toward an image plane of the collimator to where thereflected radiation beam is diverging from the image plane; comparingthe intensity of each of the first diffraction orders against the other;determining, from the comparing of the intensity of the firstdiffraction orders, the value related to the parameter of the pattern;determining a position of the target with respect to an optical axis ofthe collimator; and removing an error of relative positioning from thedetermining of the value based on the determining of the position of thetarget.
 25. The device manufacturing method according to claim 24,further comprising blocking a zeroth diffraction order from thereflected radiation beam.